Insulation system for electrical apparatus containing liquid dielectrics



July 4, 1961 J G. FORD ETAL 2,991,326

INSULATION SYTEM FOR ELECTRICAL APPARATUS CONTAINING LIQUID DIELECTRICS Filed Dec. 24, 1957 WITNESSES. INVENTORS ATTORNEY United States Patent O gm d 2,991,326 INSULATION SYSTEM FOR ELECTRICAL APPA- RATUS CONTAINING LIQUID DIELECTRICS James G. Ford, Sharon, Frank A. Sattler, Wilkinsburg, and Jack Swiss, Franklin Township, Westmoreland County, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed 'Dec. 24, 1957, Ser. No. 704,987 4 Claims. (Cl. 174-17) The present invention relates to insulated electrical apparatus containing liquid dielectrics comprising oil and has particular reference to wire enamel insulating compositions which are suitable for use in such apparatus in combination with stabilized cellulosic insulation.

Certain kinds of electrical apparatus such as trans formers contain liquid dielectrics comprising oil and cellulosic insulating materials such as paper, cotton cloth, cotton tape and wood. These cellulosic materials deteriorate rapidly at temperatures of above 100 C., particularly when in contact with oil. For this reason, electrical apparatus employing cellulosic insulation generally is not operated continually at temperatures above 105 C.

US. Patent No. 2,722,561, to McCulloch, discloses that substances like urea or non-acidic compounds derived from urea, when added to oil-filled transformers, greatly improve the thermal life of the cellulosic insulation contained therein. The patent discloses that the addition of approximately 0.3% of urea to the oil, based on the total weight of the oil, provides paper having about the same life at 125 C. as it has at 100 C. without the urea stabilizer being present. The life is measured in terms of retention of tensile strength and crease resistance.

Urea and non-acidic compounds derived from urea provide excellent stabilizing materials in oil-filled transformers employing conductors insulated with cellulosic materials. However, attempts to use, in this type of apparatus, conductors insulated with the ordinary enamel compositions have not been completely satisfactory. Thus, many enamels are attacked under certain conditions by substances like urea and its decomposition products at elevated temperatures. When polyester wire enamels, for example, are employed in a closed system containing a liquid dielectric, urea and cellulosic materials, the enamel may be dissolved or stripped completely from the wire, under some conditions of urea concentration, in a period of one week at a temperature of about 150 C.

The object of this invention is to provide electrical apparatus capable of use at temperatures above 100' C. for long periods of time, the apparatus comprising in combination, an electrical conductor with a heat hardened flexible wire enamel composition applied thereto, cellulosic insulation, oil and urea or non-acidic compounds de rived from urea.

Another object of the present invention is to provide wire insulated with a stable wire enamel composition which may be immersed at elevated temperatures for long periods of time in liquid dielectrics in contact with cellulosic insulation stabilized with urea or non-acidic compounds derived from urea.

Another object is to provide electrical apparatus including, in combination, an electrical conductor provided with a cured wire enamel insulation comprising a mixture of a glycidyl polyether and urea-aldehyde or melaminealdehyde resin, a liquid dielectric, cellulosic insulation and a stabilizer for the cellulosic insulation comprising urea or non-acidic compounds derived from urea.

Still another object of the present invention is to provide electrical apparatus including, in combination, an electrical conductor provided with a cured Wire enamel insulation comprising a mixture of a heat hardenable organosiloxane resin and a specific polyamide reaction prod- Patented July 4, 1961 ii uct, a liquid dielectric, cellulosic insulation, and a stabilizer for the cellulosic insulation comprising urea or non-acidic compounds derived from urea.

Yet another object of the present invention is to provide electrical apparatus including, in combination, an electrical conductor provided with a cured wire enamel insulation comprising a polyvinyl formal resin, aliquid dielectric, cellulosic insulation, and a stabilizer for the cellulosic insulation comprising urea or non-acidic compounds derived from urea.

Other and further objects of the invention will, in part, be obvious and will, in part, appear hereinafter.

For a more complete understanding of the present invention, reference is made to the accompanying drawing, the single figure of which is a view in elevation, partly in cross section, illustrating a transformer.

In the attainment of the foregoing objects and in accordance with the present invention there is provided electrical apparatus comprising electrical conductors in sulated with a heat hardenable flexible wire enamel composition containing the particular resins described herein. Such an enamel composition, when applied to an electrical conductor and cured, provides an insulated electrical conductor which may be employed in liquid dielectric-filled apparatus to provide a log-lived assemblage suitable for use in combination with cellulosic insulating materials associated with a stabilizer comprising urea or non-acidic compounds derived from urea.

Enamels which are suitable for use in this invention include cured heat-hardenable enamels which remain tough and flexible without softening at temperatures up to at least 250 C. It also is particularly important that the enamels be resistant to attack by alkalis or alkaline materials with which they may come in contact in the apparatus. If, for example, the urea stabilizers employed in the herein described oil-filled transformers break down or decompose during use, ammonia and/or ammonium hydroxide will result. To be satisfactory, the enamels must resist attack from this ammonia and/or ammonium hydroxide. Furthermore, the enamel must be resistant to the abrasion to which wire conductors normally are subjected during conventional coil-winding operations. Also, the enamels must be sufiiciently thermally stable to decomposition upon subjection to temperatures at least as high as 250 C.

In preparing the wire enamel compositions for this invention any one of several resins may be used. For example, resin comprising a mixture of a glycidyl polyether and urea aldehyde or melamine aldehyde resin may be used. A second resin which may be used comprises a mixture of a heat hardenable organosiloxane and a specific polyamide reaction product. Moreover, a polyvinyl formal-phenol-aldehyde resin may be employed in forming the enameled wire of the present invention.

In those wire enamel compositions wherein a resin comprising a mixture of a glycidyl polyether and ureaaldehyde or melamine-aldehyde resins are employed, the urea-aldehyde and/or melamine-aldehyde resins appear to react with and promote the curing of the glycidyl polyether or epoxide resin. It has been determined that enamels of high hardness may be obtained by additionally incorporating therein a phenol-aldehyde resin in amounts up to about 15% by weight, based on the total weight of the enamel resins. Such latter enamels are extremely resistant to the scraping and abrasion frequently encountered in coil winding and other manufacturing operations.

When it is desired to coat rectangular wire it has been found to be advantageous to incorporate a polyvinyl formal resin in the enamel composition in amounts up to about 10% to 30% by weight, based on the total weight of the enamel resins. The polyvinyl formal resin serves a tomodifythenoating properties, including increasing the viscosity of the enamel composition, whereby to permit the deposition of a satisfactorily thick enamel coating on the corners :of .the rectangular wire which coating does not pull awayfrom the corners during curing.

The resinousglycidyl ,polyethersemplpyed in the en- :amelzcompositions of the present invention may be preparedby reacting predetermined amounts of at least one polyhydric phenoland .at least one'epihalohydrin in an alkaline medium. Phenols which are suitable for use in preparingsuch resinous polymeric epoxides include those whichicontain .at least two phenolic hydroxy groups per molecule. Polynuclear phenols which have been found to be :particularly a suitable include those wherein the phenol nuclei arejoined .by carbon bridges, such for exampleias 4,4-dihydroxy.-diphenyl-dimethylmethane (referred tohereinafter as bis-phenol A) and 4,4'-dihydroxy-diphenyl-methane. In admixture with the named polynuclear phenols, use also may be made of those polynuclear phenolswherein the phenol nuclei are joined by sulfur bridges such, for example, as 4.4'-dihydroxvdiphenyl-sulfone.

While it is preferred to use epichlorohydrin as the epihalohydrin in the preparation of the resinous polymeric epoxide starting materials of the present invention, homologues thereof, for example, epibrornohydrin and the-like also may be used advantageously.

In the-preparation of the resinous polymeric epoxides, aqueous alkali is employed to combine with the halogen of the epihalohydrin reactant. The amount of alkali employed should be substantially equivalent to the amount of halogen present and preferably should be employed in an amount somewhat in excess thereof. Aqueous mix tures of alkali metal hydroxides, such as potassium hydroxide and lithium hydroxide, may be employed although it is' preferred to use sodium hydroxide since it is relatively inexpensive.

The resinous polymeric epoxide, or glycidyl polyether of a dihydric phenol, suitable for use in this invention has a l;2'-epoxy equivalencygreater than 1.0. By epoxy equivalency reference ismade to the average number of 1,2-ep oxy groups contained in the average molecule of the glycidyl-ether. Owing to'the method of preparation of the glycidyl polyethers and the fact that they are ordinarily a mixture of chemical compounds. having somewhat different molecular weights and contain some compounds whereinthe terrninalglycidyl radicals are in hydrated form, the epoxy equivalency ofthe product is not necessarily the integer 2.0. However, in all cases it is a value greater than 1.0. The 1,2-epoxy equivalency of the polyethers is thus a value between 1.0 and 2.0.

Resinous polymeric epoxides or glycidyl polyethers suitable for use in accordance with this invention may be prepared by'adrnixing and reacting from one to two mol proportions of epihalohydrin, preferably epichlorohydrin, with about one mol proportion of bis-phenol A in the presence of at least a stoichiometric excess of alkali based on the amount of halogen.

To prepare the resinous polymeric epoxides, aqueous alkali, bis-phenol A-and epichlorohydrin are introduced into and'admixed in areaction vessel. The aqueous alkali servesto dissolve the bis-phenol A" with the formation ofthe alkali salts thereof. If desired, the aqueous alkali and bis-phenol A may be admixed first and then the -,epichlorohydrin added thereto, or an aqueous solution of alkaliand bis-phenol A may be added to the epichlorohydrin. In any case, the mixture is'heated in the vessel to a temperature within the range of about 80 C. to 110'C. for a-period oftimevarying from about one-half 4 I hour to three hours, or more, depending upon the quantities of reactants used.

Upon completion of heating, the reaction mixture separates into layers. The .upper aqueous layer is withdrawn anddiscarded, and the lower layer is washed-with hot water to remove unreacted alkali and halogennsalt, in this case, sodium chloride. If desired, dilute-acids, for example, acetic acid or hydrochloric acid,.may -.be employedduring the washing procedure to neutralize the excess alkali.

Glycidyl polyethers may be prepared in either-so1id or liquid form. The commercially available glycidyl polyethers which aresolids are less expensive than the liquid grades, thus the use of the solid materials affords a substantial cost savings when used in accordance with this invention. Usually, the liquid polyethers have very few, if any hydroxyl groups. The solid polyethers, on the other hand, have a substantial number of hydroxyl groups per molecule.

vThe followingexample illustrates the preparation of :a glycidyl polyether suitable for use in this invention.

EXAMPLE I .Into a suitable reaction vessel there was introduced two litersof water, 160 parts'ofsodium hydroxide and 685 parts of bis-phenol A. This mixture was agitated for several minutes at a temperature of about 50 C. Thereafter, a mixture of approximately 280 parts of epichlorohydrin and parts of hi-flash naphtha was addedto the reaction-mixture. This formulation contains a'ratio of one mol of epichlorohydrin to one mol of bis-phenol A, with about 15% of sodium hydroxide and about 10% of solvent. Thereaction mixture was maintained at a temperature of about C. to C. for aperiodrof about two hours. The resin so produced was washed free of alkali-and salt by:successive washings with hot water at 100 C. After four separate washings the solution was neutralized Withsulphuric acid and washed once with water. .As much water as possible was removed from the kettle by decantation and the resin was dried by heating tol50 C. The hot resin was then poured into a pan and permitted to cool.

A typical urea-aldehyde resin suitable for use with the glycidyl polyether so prepared is the reaction product of one mol of urea with two mols of an aldehyde such as formaldehyde. Ordinarily, the reaction is carried out in analcohol such as butanol, and the reaction product is an alcoholated urea-resin, for instance, butylated ureaaldehyde resin. The preparation of urea-aldehyde resins is well known in the art'and need not be detailed herein. When preparing the enamel compositions of this invention, the clarified and dehydrated urea-aldehyde in the solvent soluble stage or A-stage is dissolved in a suitable solvent or mixture of solvents and blended with the other enamel components.

The melamine-aldehyde resins which may be used with the glycidyl polyethers may be prepared by reacting from two to six mols of an aldehyde, such as form-aldehyde, with one mol of melamine under alkaline conditions. The number of substituted methylol groups occurring in the product is generally dependent upon the mol ratio of the reactants. Thus, it is possible to react formaldehyde or another aldehyde with melamine in mol ratios sufiicient to give mono-, di-, tri-, tetra-, penta-, and hexamethylol melamines or mixtures thereof. Any of these derivatives or mixtures thereof are satisfactory in preparing the resins of this invention.

Aldehydes other than formaldehyde may be used in preparing the urea-aldehyde and/or melamine-aldehyde resins of this invention. For example, acetaldehyde, propicnic aldehyde, butyric aldehyde, benzaldehyde, and the like may be used to replace a part or even all of the formaldehyde, with satisfactory results being obtainable.

Another resinous composition suitable for use in preparing wire-enarnels for this invention may be prepared by admixing (A) from 1 part to 50 parts by weight of a heat-hardenable organosiloxane resin with (B) 100 parts by Weight of a specific polyarnide reaction product. This resin and enamels prepared therefrom are described in copending application Serial No. 634,275, filed January 15, 1957, which application is assigned to the same assignee as the present invention.

The organosiloxane resins that have been found particularly satisfactory are the phenyl methyl polysiloxanes having from 1.3 to 1.95 phenyl and methyl groups per silicon atom. Particularly good results are obtained with organosiloxanes having a total of from 1.5 to 1.8 phenyl and methyl groups per silicon atoms. It has been found that the phenyl groups may be replaced with up to 50% of diphenyl groups with equally good results. It will be understood that other organic groups other than phenyl and methyl groups may be present in the siloxane. Thus, the phenyl methyl organosiloxanes may comprise a small proportion, for example, up to mol percent of other organic groups such as tolyl, allyl, ethyl and the like.

The polyamide employed in preparing the resinous composition of this invention has the generic formula where mrepresents an integer having a value of two or more, and n is 0 or an integer. R represents the residue of an ethylenically unsaturated dicarboxylic acid after removal of the hydroxyl groups. due of isophthalic acid, terephthalic acid, or an aliphatic dicarboxylic acid with the hydroxyl groups removed, said aliphatic dicarboxylic acid having no ethylenic unsaturation and having at least two and not more than eight noncarboxyl carbon atoms.

The polyamide is derived by admixing and heating to reaction (a) at least one acidic compound selected from the group consisting of ethylenically unsaturated dicarboxylic acids and anhydrides thereof and (b) at least one diamino compound selected from the group consisting of primary hydrocarbon diamines and polyamide reaction products derived by reacting a primary hydrocarbon diamine and at least one dicarboxylic acid having terminal amino groups, said acid being selected from the group consisting of isophthalic acid, terephthalic acid and saturated aliphatic dicarboxylic acids having from 2 to 8 noncarboxyl carbon atoms. For each mol of diamino compound there is provided from about 1 mol to 2 mols of the acidic compound.

Examples of suitable ethylenically unsaturated dicarboxylic acids and anhydrides thereof are maleic acid, maleic anhydride, fumaric acid, citraconic acid and citraconic anhydride. It will be understood that esters of the above-mentioned acids and anhydrides may be substituted for all or a part of the acids in the reaction with the cliamino compound. For example, dibutylmaleate may be substituted for all or a part of maleic acid.

The primary hydrocarbon diamines employed should have at least two and no more than six methylene groups. Examples of suitable primary diamines are ethylene diamine, 1,3-propane diamine, 1,4-butane diamine and hexamethylene diamine.

'I he polyamide reaction products having terminal amino groups that may be employed in place of the above-mentioned primary diamines are those derived by reacting an excess of at least one of the primary diamines mentioned above with an aliphatic dicarboxylic acid or anhydride thereof having at least two and no more than eight noncarboxyl carbon atoms and having no other reactive groups than the carboxyl groups or anhydride groups. Examples thereof are adipic, succinic and glutaric acids. The above-mentioned dicarboxylic acids may be replaced in whole or in part by isophthalic acid or terephthalic acid.

The preparation of the above-mentioned polyamide re- R represents the resiaction products is carried out by heating the mixture in a non-reactive solvent. To insure the presence of ter-, minal amino groups in the reaction product, there is employed from about 0.1 mol to 0.9 mol of dicarboxylic acid for each mol of the primary hydrocarbon diamine. In the preparation of the above polyamides it is preferred to employ a high boiling solvent such as cresol, dichlorobenzene or the like. These solvents do not enter into the reaction but provide a common solvent for the reactants and further prevent the primary diamine from evaporating from the mixture when the mixture is heated. When polyamides with terminal amino groups are employed as the diamino compound to prepare the polyamide used in the resinous composition of this invention, it is preferred to employ those having an average molecular weight of not in excess of 1500.

The polyamides, prepared as just described, when admixed and reacted with the organosiloxanes, form resinous compositions which are dissolved in one or more solvents such as butanol, xylene, dichlorobenzene or the like to provide enamels which can be applied to wire and cured to produce hard tough enamel coatings.

The polyvinyl formal resins which are suitable for incorporation in solvents to provide enamels for use in this invention may be prepared, for example by hydrolyzing polyvinyl acetate and then reacting the resulting hydrolyzate with a suitable aldehyde. A suitable wire enamel is one containing about 50% to 67% by weight of polyvinyl formal and 50% to 33% by weight of phenol aldehyde resin.

In preparing a wire enamel composition of the present invention using an epoxy resin and a ureaor melaminealdehyde resin, a potentially reactive, solvent soluble glycidyl polyether resin should be employed in an amount within the range of about 50% to by weight, the balance comprising potentially reactive, solvent soluble, urea formaldehyde and/ or melamine formaldehyde, alone, or further modified with phenol-aldehydes and/or polyvinyl formal resins. The enamel composition may be prepared by dissolving the partly reacted resinous materials in mixtures of hydrocarbon solvents, for example, aromatic hydrocarbons such as benzene, toluene, xylene, or any of the higher boiling aromatic hydrocarbons with an aliphatic ketone such as methylethyl ketone, methyl propyl ketone, methylisopropyl ketone, methylisobutyl ketone, and the like. Similarly, mixtures of any one of the aforementioned aromatic hydrocarbons may be employed with diacetone alcohol to give excellent vehicles for the resin solids of the present enamel compositions.

In addition to the aforementioned solvents other materials which are solvents for the resin solids may be used. Suitable examples include Cellosolve acetate, methyl Cellosolve acetate, cresol, and the like. It is believed that when enamel comprising the mixture of resins is applied to the electrical conductor and heated a reaction product comprising a copolymer of the glycidyl polyether and the urea-formaldehyde and/or melamine-formaldehyde resin is produced. However, we do not wish to be limited to any particular theory of the actual reaction forming the cured reaction product.

The wire enamel compositions herein disclosed and claimed, when baked on electrical conductors, exhibit a high degree of hardness and toughness, while retaining surprising flexibility, and have such outstanding chemical resistance that they are almost completely unaffected by various solvents'such as alcohol, toluene, oils and the like.

More particularly, however, the cured wire enamel compositions are highly durable in liquid dielectrics in which is present urea or its derivatives for stabilizing the cellulosic insulation.

It will be appreciated that while other wire enamel compositions may be prepared, without excessive effort. that will exhibit high values as to one or even several of the critical properties such as hardness, toughness or solvent resistance desired in a good enamel, some other property 'or properties'willbe-so poor that such composition willnot 'be'satisfactory in an properties for-commercial usein electrical apparatus containing liquid dielectrics and urea stabilized cellulosic insulating material.

Those skilled in the'art are-well acquainted with the extraordinary difliculties involved in producing a wire enamel of which all the'properties are betterthan fair for usein liquid dielectricsin oil-filled transformers.

The wire enamel'compositions of this invention may be applied not only to bare copper wire, but they may be applied to cotton "covered wire, asbestos covered wire, glass *fiber covered-wire, and wire-carrying other fibrous orceliulosic materials toprovi'de for good electrical insulating coatings. The fibrous'materials may be applied before or after the enamel is applied, or concurrently therewith, as desired.

Electrical conductors insulated with the enamels described hereinabove are suitable'for use'in-oil-filled transformers containing cellulosic insulation stabilized with urea'or'non acidic compounds derivedfrom urea. The non-acidic compounds have the formula wherein R isselected. from the groupconsisting ofhydrogen, monovalent 'hydrocarbon radicals, -.CN radicals, and methylol radicals,.and R'is selected from the. group consisting of oxygen, sulfur, hydrogen, and -NH radicals, there being a total of atleast two hydrogen atoms attached directly tothe C and N atoms, to reduce greatly the rate of loss off-strength of the cellulosic insulation. Examples of the .stabilizingcompounds suitable for this purpose are urea, substitution derivatives of urea, and reaction products :of urea and aldehydes such as ureaformaldehyde resins. Excellent results have been had using thealkyl substitution derivatives in which one or more aliphatic hydrocarbon groups are substituted for hydrogen in both urea and thiourea, for example, methyl, ethyl, butylpropyl, amyl and octyl radicals. Examples of such compounds are thiourea, hexamethylenetetramine, guanidine carbonate, tertbutyl urea, tertamyl urea, n-butyl urea, .l,1-diethyl urea, .l,3-diethy-l thiourea, 1,3-diisopropyl thiourea, l,3-dimethyl urea, 1,3-dibutyl urea, dicyanidiamide, methplol urea and biuret.

The term non-acidiccompounds, as used inthepresent specification and-claims, refers to compounds which when .placed .in water .in amounts of about 1% and warmed for afew hours produce aqueous solutions of a .pHof 7 or higher. Thus, water with 0.2% by weight of ureahas apH of approximately 10. Dicyanidiamide under the same .conditionsgives a pH of 8, while thiourea gives a solution having a pH of 7. On the other hand, dithiobiuret gives a pH of and uramil a pH of about 1, and each of these latter have been found to be useless in retarding the rateof deterioration of cellulosic materials in liquid dielectrics.

In order to increase the thermal stability of cellulosic insulation in contact with liquid dielectrics, thereby greatly reducing its rate of loss of strength, a small quantity of the stabilizing compound is introduced into the liquid dielectric or otherwise placed in contact with the liquid dielectric. The quantity of the compound may be quite small. For example, a quantity equal to 0.01% by weight of the liquid dielectric is sufiicient; however, a larger amount does no harm and may bedesirable in some cases. One or more of the compounds may be present in an amount of up to of the weight of the dielectric. The compound maybe finely divided. The compound need not be appreciably soluble in the dielectric, and in fact it was found that even though urea is only very slightly :solublein cold oil, it produced excellent results.

8 At higher-temperatures asffor example, at C., urea will melt and disperse readily in oils.

The-stabilizer for the'cellulosiclinsulation also may be placed'in the paper either added to: the furnish in the paper mill'so thatthe'paper-contains urea'in-the fiber structure, or is-sifted or otherwise applied as the paper is wrapped on the conductors or coils, in the oil, or in both. Since the solubility of substances like urea and non-acidic compounds derived therefrom is very slight, less than 0.1 in cold transformer oil, and since oil circulation through tightly wound coils is very slow, it is particularly advantageous to incorporate the stabilizer in the paper itself at some stage during the manufacture of'the paper. Addition of about 3% of urea or derivative thereof, .based on the weight of the'paper, provides satisfactory stabilizing results.

Referring to the drawing, there'is illustrated a-transformer in accordance with the present invention. The transformer comprises a tank 10 carrying 'a support 12 internally on whichmagnetic core 14 and a coil 16 is disposed. Coil 16 comprises a high voltage winding'18 and a low voltage winding 20 each insulated withthe wire enamel composition of the present invention. The windings also are insulated from one another by cellulosic insulation 22 which comprises paper, cotton, 'or other cellulosic material. An exterior cellulosic wrapping 24 of cloth or paper may be applied to the coil 16. In some cases, pressboard, wood,.or cardboard spacers or various other cellulosic products may be applied to the electric windings. A liquid dielectric 26is disposed within the tank 10 to cover the core 14 and coil 16 in order to 'insulate them and to dissipate the heat produced in operation. The liquid dielectric may be any suitable dielectric such as oil.

In applying the enamel compositions on electrical .conductor wire to be used in the apparatus of the present invention it has been found that an enamel coating composition having a resin solids content of from 15% to 30% .issuitable for use in most enamelling machines. 'If enamel coating dies are employed, a more viscous solution having a. higher resin solids content may .beemployed, for example, a 50% solids content. The wire enamel so prepared may be applied by a flowing or by a dipping process or other suitable method of applying a film or a coating of .the enamel solution to an electrical conductor. To drive ott the solvent and to cure or copolymerizetthe enamel compositiomthe coated wiresare heated or baked. The baking temperature may be varied within a wide range, .since for most satisfactory .applicationithe temperature within the tower adjacent the wire must be correlated with the speed of the wire through the enamelling tower as well as the size .of'the tower. Thus, in a small high speed tower, a temperature of approximately 450 C..may be satisfactory, while in a largertower with slower wire travel speeds a temperature of about'250 "C. may be more suitable.

In orded to indicate more fully the advantages "and capabilities of the present invention the following specific examples are set forth. The parts given are by weight unless otherwise indicated.

EXAMPLE II An enamel composition was prepared by dissolving 75 parts of the glycidyl polyether prepared as described in Example I in parts of methyl Cellosolve and 125 parts of'xylene. This solution was mixed with 50 parts of a butylated urea-forrnaldehyde solution containing 50% solids in a mixture composed of 30 parts of butanol and 20 parts of xylene. The wire enamel so prepared was coated 1.5 mils thick on number 17 A.W.G. copper wire at a rate of 28 feet per minute at 430 C., in a 15 foot coating tower. The enamel composition coated smoothly and had good flexibility over a wide baking range. Twisted samples of wire coated at this coating rate and temperature were tested for thermal stability according to American Institute of Electrical Engineering Test No. 57 at 200 C. The thermal life of this enamel as determined from an average of samples tested was 336 hours at 200 C. The enamel had a scrape hardness of 40-43 ounces using a 9 millimeter diameter knife edge when tested in the tester described in US. Patent 2,372,093, and had a dielectric strength of from 4,500 to 6,500 volts.

EXAMPLE III A wire enamel composition was prepared by dissolving 65 parts of the glycidyl polyether prepared as described in Example I in 130 parts of methyl Cellosolve acetate and 130 parts of xylene. This solution was blended with 70 parts of a butylated urea-formaldehyde solution con taining 50% solids dissolved in a mixture composed of 30 parts of butanol and parts of xylene. This wire enamel was coated on No. 17 A.W.G. copper wire at an average rate of feet per minute at 430 C. Twisted samples of wire coated at these various coating rates and temperatures were tested for thermal stability according to American Institute of Electrical Engineering Test No.'

57 at 200 C. The thermal life of this enamel was 436 hours at 200 C. The enamel coated smoothly and had good flexibility. The enamel had a scrape hardness of from 39 to 44 and exhibited dielectric strengths as high as 9,000 volts.

EXAMPLE IV To 75 parts of the glycidyl polyether prepared as described in Example I dissolved in 150 parts of cresol and 125 parts of xylene was added 50 parts of butylated urea-formaldehyde solution containing 50% solids in parts of butanol and 20 parts of xylene. The enamel was coated on No. 17 A.W.G. copper wire at a rate of 25 feet per minute at 430 C. The cured enamel had a scrape hardness of from 39 to 42 ounces and had a dielectric strength varying from 4,000 to 9,500 volts. A wire enamel of essentially the same characteristics is obtained when 275 parts of methyl Cellosolve acetate is used in place of the cresol-xylene solvent.

Example IV Was repeated increasing the urea-formaldehyde resin content to 70 parts, on a solids basis, and decreasing the glycidyl polyether content to 65 parts. This had the eifect of increasing the scrape hardness of the cured enamel to 45 to 52 ounces without adversely affecting any of the other properties.

ing (1) 20 parts of a polyvinyl formal resin of medium molecular weight dissolved in 225 parts of cresol, (2) 52 parts of the glycidyl polyether of Example I dissolved in 104 parts of methyl Cellosolve acetate and 310 parts of xylene, and (3) 56 parts of a solids basis, urea-formaldehyde resin solution dissolved in 30 parts of butanol and 2.0 parts xylene. This wire enamel composition was coated on No. 17 A.W.G. copper wire at a rate of 28 feet per minute at 430 C. The cured wire enamel had excellent flexibility characteristics and excellent heat shock resistance. The enamel had a scrape hardness of 53 to 54 ounces and a dielectric strength varying from 6,000 to 11,000 volts. This enamel is particularly well suited for coating on rectangular wire.

EXAMPLE VI An enamel was prepared by dissolving (1) 58.5 parts of the glycidyl polyether of Example I, (2) 63 parts of a 50%, solids basis, urea-formaldehyde solution in 30 parts of butanol and 20 parts of xylene, and (3) 10 parts of a polyvinyl formal resin of medium molecular weight in 117 parts of methyl Cellosolve acetate. parts of cresol, and 83 parts of xylene. This wire enamel was coated on No. 20 A.W.G. copper wire at the rate of 28 feet per minute at 430 C. The enamel had a scrape hardness of from 34 to 37 ounces and a dielectric strength of from 5,500 to 10,500 volts. This wire enamel com;

position also was particularly well suited for coating on rectangular wire.

EXAMPLE VII The following were introduced into a closed reaction vessel provided with a condenser:

Parts Hexamethylenediamine, 80% aqueous solution 310 (2.0 mols) Cresol 200 Isophthalic acid 200 (1.2 mols) completed, 273 parts (1.2 mols) of dibutylmaleate dissolved in 250 parts of cresol were added to the mixture. The mixture was then heated to approximately C., which temperature was maintained for about 1% hours. The temperature of the mixture was then gradually increased to C. during the succeeding two hours, during which time between 60 parts and 80 parts of butanol distilled off.

One hundred parts of a 60% solution of phenyl-diphenyl-methyl siloxane resin in xylene were then added to the mixture to form a resinous composition. The R to Si ratio was about 1.5 and the phenyl, diphenyl and methyl groups were present in approximately equal proportions. The resinous composition was then diluted by.

adding 300 parts dichlorobenzene and 600 parts of butyl alcohol.

No. 13 (A.W.G.) copper wire was coated with six dips in the diluted resinous composition in a wire enameling tower having an oven maintained at 825 F. The cured resin enamel on the wire was smooth and hard, and it had a scrape test value of from 45 to 55 ounces as determined on the scrape tester described in Patent 2,372,093. The enameled wire successfully passed the quick jerk and heat shock tests, and it showed high resistance to hot impregnating varnishes and varnish solvents such as toluene. The enameled wire was wound around a mandrel having the same diameter of the enameled wire and was aged for four weeks at 200 C. without cracking or otherwise exposing the copper wire.

EXAMPLE VIII A wire enamel containing a polyvinyl formal-phenolaldehyde resin may be prepared according to the method disclosed in US. Patent No. 2,307,588, which disclosure is incorporated herein. Briefly, such a wire enamel may be prepared by mixing 16% by weight of resins and 84% by weight of solvents. cresol-formaldehyde resin and 10.5% polyvinyl formal and the solvent includes about 25.5% cresol and 58.5%

' naphtha.

The data set forth in the following table illustrate the improved results obtainable when employing the insulation system of the present invention as compared with other systems. The data was obtained from tests run The resins include about 5.5%v

the" three coils was placed in'a transformertank'partially filled with'transformer -oil. Thecoilsffwere oompletely covered by the oil in each tank. A coverwas placed on each tank and currentwas applied to each coil in an amount sufficient to generate a coil .hot spot temperaature of 150 C. This temperature was'maintained for five days, the duration of thetest.

The test data show that-in coil I, employing .plain kraft paper with polyester-polyann'de enamel, the enamel was not degra'dedbut that'the paper was degraded as evidenced by a greatly reduced breakingstrength. "In coil II, where urea was added to the paper, the paper was greatly'improved' but the,polyester-polyamideenamel waspractically completely dissolved off-the "wirein deep coilportions. When a combination ofurea, paper and glycidyl polyether-urea-formaldehyde enamel was 'used,

coil 111, both the paper and enamel were -satisfactory after the accelerated test and sufiered no significant-deterioration.

Smallest mandrel diameter bend without oraekingof enamel While the present invention has been described with respect to the insulation of an oil-filled transformer it will be understood by those skilled in the art that it is equally applicable to other electrical apparatus such capacitors, reactors, cables, switchgear and the like in which liquid dielectrics are in contact with cellulosic i-nsulation and with electrical conductors provided with enamel insulation.

We claim as our invention:

1. In electrical apparatus, the combination comprising an electrical conductor winding developing heat during use of the apparatus, cellulosic insulation present in the apparatus and in contact with the winding and normally being subject to loss of physical strength with passage of time at the temperatures developed, a liquid dielectric consisting essentially of petroleum oil applied to the winding and to the cellulosic insulation to dissipate the heat developed therein, and, in the liquid dielectric to maintain the physical strength of the cellulosic insulation, a non-acidic compound having the formula attached directly to the C and N atoms, the electrical conductor winding being provided with a hard, tough, flexible enamel coating which is resistant to attack by ammonia, ammonium hydroxide, and other decomposition products of said non-acidic compound and which does not soften at temperatures up to about 250 C., the

coating comprising a heat-hardened resin selected from the group consisting of (A) from 50 to parts by weight of'a glycidylpolyether of a dihydric phenol having-a 1,2-epoxy equivalency greater than 1 and from 50 to 15 parts by weight of at least one thermosetting resin selected from the group consisting of melamine-aldehyde and urea-aldehyde resins.

(B) (l) parts by weight of a polyamide resin composition derived by admixing and heating (i) from about 1 to 2 mole of an unsaturated acidic compound selected from the group consisting of maleic acid, maleic anhydride, fumaric acid, and the monomethylsubstitution derivatives for the non-carboxyl hydrogen thereof for (ii) one mol of a diamino compound selected from the group consisting of primary hydrocarbon diamines having from 2 to 6 carbon atoms and polyamide reaction products having terminal amino groups and having an average molecular weight ofnot in-excess of 1500, and from (2) one part to SOparts by weight of a solvent soluble, heat-hardenable organosiloxane resin having from 1.3 to 1.95 hydrocarbon groups per silicon atom, and

(C) from 50% to 67% by weight of polyvinylformal and 50% to 33% by weight of phenol aldehyderesin.

2. In electrical apparatus, the combination'comprising an electrical conductor winding developing heat during use of the apparatus, cellulosic insulation present in the apparatus and in contact with the winding and normally being subject to loss of physical strength with passage of time at the temperatures developed, a liquid dielectric consisting essentially of petroleum oil applied to the winding and to the cellulosic insulation to dissipate the heat developed therein, and, in the liquid dielectric to maintain the physical strength of the cellulosic insulation, a nonacidic compound having the'iormula where R is selected from the group consisting of hydrogen, monovalent hydrocarbon radicals, --CN radicals, and methylol radicals and R is selected from the group consisting of oxygen, sulfur, hydrogen, and --NH radicals, there being a total of at least two hydrogen atoms attached directly to the C and N "atoms, the electrical conductor winding being provided with a hard, tough, flexible enamel coating which is resistant to attack by ammonia, ammonium hydroxide, and other decomposition products of said non-acidic compounds, and which does not soften at temperatures up to about 250 C.-, the coating comprising from 50 to '85 parts by weight of a glycidyl polye'ther of a dihydric phenol having a 1,2- epoxy equivalency greater than 1 and from 50 to 15 parts by weight of at least one thermosetting resin selected from the group consisting om melamine-aldehyde and ureaa-ldehyde resins.

3. In electrical apparatus, the combination comprising an electrical conductor winding developing heat during use of the apparatus, cellulosic insulation present in the apparatus and in contact with the winding and normally being subject to loss of physical strength with passage of time at the temperature developed, a liquid dielectric consisting essentially of petroleum oil applied to the winding and the cellulosic insulation to dissipate the heat developed therein, and, in the liquid dielectric to maintain the physical strength of the cellulosic insulation, a non-acidic compound having the formula where R is selected from the group consisting of hydrogen, monovalent hydrocarbon radicals, CN radicals, and methylol radicals and R is selected from the group consisting of oxygen, sulfur, hydrogen, and NH radicals, there being a total of at least two hydrogen atoms attached directly to the C and N atoms, the electrical conductor Winding being provided with a hard, tough, flexible enamel coating which is resistant to attack by ammonia, ammonium hydroxide, and other decomposition products of said non-acidic compounds, and which does not soften at temperatures up to about 250 C., the coating comprising (1) 100 parts by weight of a polyamide resin composition derived by admixing and heating (1') from about 1 to 2 mols of an unsaturated acidic compound selected from the group consisting of maleic acid, maelic anhydride, fumaric acid an the monomethyl substitution derivatives for the non-carboxyl hydrogen thereof for (ii) each one mol of a diamino compound selected from the group consisting of primary hydrocarbon diamines having from 2 to 6 carbon atoms and polyamide reaction products having terminal amino groups and having an average molecular weight of not in excess of 1500 and from (2) one part to 50 parts by weight of a solvent soluble, heat-hardenable organosiloxane resin having from 1.3 to 1.95 hydrocarbon groups per silicon atom.

4. In electrical apparatus, the combination comprising an electrical conductor winding developing heat during use of the apparatus, cellulosic insulation being present in the apparatus and in contact with the winding and normally being subject to loss of physical strength with passage of time at the temperatures developed, a liquid dielectric consisting essentially of petroleum oil applied to the Winding and to the cellulosic insulation to dissipate the heat developed therein, and, in the liquid dielectric to maintain the physical strength of the cellulosic insulation, 3 non-acidic compound having the formula t l y R R where R is selected from the group consisting of hydrogen, monovalent hydrocarbon radicals, --CN radicals, and rnethylol radicals and R is selected from the group consisting of oxygen, sulfur, hydrogen, and NH radicals, there being a total of at least two hydrogen atoms attached directly to the C and N atoms, the electrical conductor winding being provided with a hard, tough, flexible enamel coating which is resistant to attack by ammonia, ammonium hydroxide, and other decomposition products of said non-acidic compound, and which does not soften at temperatures up to about 250 C., the coating comprising from to 67% by weight of polyvinyl formal and 50% to 33% by weight of phenol aldehyde resin.

References Cited in the file of this patent UNITED STATES PATENTS 2,307,588 Jackson et al. Jan. 5, 1943 2,591,539 Greenlee Apr. 1, 1952 2,637,716 Ott May 5, 1953 2,722,561 McCulloch Nov. 1, 1955 2,730,466 Daszewski Jan. 10, 1956 2,730,467 Daszewski Jan. 10, 1956 2,823,195 Shorr et al Feb. 11, 1958 2,864,728 Predota et al. Dec. 16, 1958 FOREIGN PATENTS 542,874 Great Britain J an. 30, 1942 700,708 Great Britain Dec. 9, 1953 707,320 Great Britain Apr. 14, 1954 

1. IN ELECTRICAL APPARATUS, THE COMBINATION COMPRISING AN ELECTRICAL CONDUCTOR WINDING DEVELOPING HEAT DURING USE OF THE APPARATUS, CELLULOSIC INSULATION PRESENT IN THE APPARATUS AND IN CONTACT WITH THE WINDING AND NORMALLY BEING SUBJECT TO LOSS OF PHYSICAL STRENGTH WITH PASSAGE OF TIME AT THE TEMPERATURES DEVELOPED, A LIQUID DIELECTRIC CONSISTING ESSENTIALLY OF PETROLEUM OIL APPLIED TO THE WINDING AND TO THE CELLULOSIC INSULATION TO DISSIPATE THE HEAT DEVELOPED THEREIN, AND, IN THE LIQUID DIELECTRIC TO MAINTAIN THE PHYSICAL STRENGTH OF THE CELLULOSIC INSULATION, A NON-ACIDIC COMPOUND HAVING THE FORMULA 